1. Field of the Invention
The invention relates generally to a method and a circuitry arrangement for operating a brushless electric motor which has a permanent magnet rotor and a stator with three winding strands electrically offset by 120°. More specifically, the invention relates to an electric motor for driving a dental treatment instrument, for example a drill handpiece, or a dental-technical instrument.
2. Related Technology
Dental or dental-technical hand instruments normally have treatment tools which, with the aid of a drive, are set into oscillation or rotation. In particular with dental treatment instruments there is the possibility of configuring the drive in the form of an air motor or a turbine, or of using an electric motor. Turbines are normally more economical and space-saving; however an electric motor has with regard to its operational characteristics certain advantages. Thus an electric drive offers for example the possibility of varying the speed of rotation of the treatment tool and the torque exercised by the treatment tool. Through this the treatment instrument can be used more flexibly. Further, treatments can be carried out with a higher precision.
As motors for use in dental treatment instruments so-called brushless motors have in particular have proved to be suitable in the past. There are involved here motors having a permanent magnetic rotor which is surrounded by a stator having a plurality of windings. The stator normally has three winding strands electrically offset by 120°, to which there is delivered a supply voltage depending upon the disposition of the rotor. By appropriate switching-over (so-called “commutation”) of the supply voltage for the stator windings the rotor is set into rotation by the rotating magnetic field thereby arising.
Such brushless electric motors, which are often described as BLDC (brushless direct current) motors, stand out inter alia due to their high performance efficiency. For this, however, it is a prerequisite that the commutation of the supply voltage for the stator windings is effected synchronously with the rotation of the rotor. Thus a detection of the angular position of the rotor is required, which could be effected by the use of appropriate sensors, in particular so-called Hall sensors. Since however additional components would be required for this, in the meantime the rotor disposition is also detected via the voltage induced in the stator windings, the so-called electromotive force (EMF), by the rotor. This voltage depends for the individual windings respectively on the dispositions of the magnet or the magnets of the rotor to the corresponding winding strand, whereby in particular the time point at which the voltage induced in a winding is equal to zero indicates a particular position of the rotor, which is taken into account for the derivation of a suitable commutation time point. Thus for example the commutation is normally carried out at a time point at which, after passing through the voltage zero-crossing, the rotor has turned by about a further 30°. By monitoring the EMF, the commutation of the supply voltage thus can be adapted to the rotor rotation in a suitable manner. Further, the speed of rotation of the motor also can be determined in this way.
FIG. 5 shows a circuitry arrangement known from the state of the art, which has been used to date for the detection of the EMF voltage zero-crossings for a brushless motor. To date, in principle, in this procedure either an artificial so-called star point was generated and the current-free phase to be examined compared against this artificial star point. As an alternative to this the star point of the motor could also be taken and directly delivered to a comparator.
FIG. 5 shows a solution in accordance with the first variant, as known for example from DE 100 23 370 A1, and in which, on the one hand, the voltages of the phases U, V and W arising at the three winding strands of the stator windings are respectively delivered to the (positive) inputs of three comparators IC1, IC2 and IC3 via three resistances R1, R2, and R3, and on the other hand are brought together to a common star point S via three resistances R4, R5, and R6. At the respective second inputs of the comparators IC1 to IC3 there is then applied the star point voltage arising in the star point S.
The three so-called star point resistances R4 to R6 have the same resistance value in this known circuitry arrangement, which has the consequence that the star point voltage can be calculated as follows:UStarpoint≈⅓UEMF.PhaseNN+½UZK 
Here, UEMF.PhaseNN corresponds to the generator voltage of the motor at the phase (U, V or W) currently being examined, UZK corresponds to the intermediate circuit voltage with which the stator windings are operated, while in contrast UStarpoint corresponds to the voltage at the artificial star point S.
The respectively active comparator IC1, IC2, or IC3 then calculates the difference voltageΔU=UEMF.PhaseNN−UStarpoint orΔU=UEMF.PhaseNN−⅓UEMC.PhaseNN−½UZK 
and switches when the voltage difference exceeds a determined hysteresis voltage. The voltage zero crossing detected in this way gives information about the disposition and the speed of rotation of the rotor.
This known circuitry arrangement in accordance with FIG. 5 has proved itself in practice and functions without problems in particular if the intermediate circuit voltage is stable and is not chopped by pulse width modulation. Thus no clocked final stage need be used for the motor but instead a regulated intermediate circuit is required.
If, however, the motor speed is regulated in that the motor voltage is produced by clocking the final stage, that is by pulse width modulation, this quickly leads with the circuitry arrangement known from the state of the art to disturbances and with that to faulty commutation of the motor. The reason for this is that false voltage zero-crossings are detected which arise due to voltage spikes which are produced by the pulse width modulation and have as consequence a false switching of the comparator. A further problem of the known circuitry arrangement in accordance with FIG. 5 consists in the fact that, upon a certain minimum speed of rotation of the motor being undershot, this no longer provides useable results, since the signals here obtained are too weak to unambiguously detect a voltage zero-crossing.